It is known that a grain oriented silicon steel strip exhibits a high degree of orientation of grains in a direction of an axis &lt;001&gt;, in which direction the steel strip can be easily magnetized, and therefore, is a magnetic material useful for producing steel cores of motors or transformers.
The grain oriented silicon steel strip is usually produced by a process comprising a steel-making step, hot rolling step, cold rolling step, decarburization step and finish annealing step. In the finish annealing step, the decarburized silicon steel strip which has been converted into a plurality of strips having a desired shape, is annealed in a reduction atmosphere at a temperature of from 1100.degree. to 1300.degree. C. In order to feed the steel strip to the finish annealing step, the pieces are coated with an annealing separator so as to prevent them from sticking to each other. Usually, the annealing separator comprises a conventional type of magnesium oxide. The magnesium oxide can exhibit an excellent resistance to heat. The annealing separator is not only effective for preventing the pieces of the steel strip from sticking to each other, but also, is influential in secondary recrystallization of the grains and in formation of a glassy insulating film on the surfaces of the steel strips during the finish annealing operation. Therefore, in order to produce a finish annealed silicon steel strip of excellent quality, it is important that the annealing separator have a high level of quality. In order to obtain the high level of quality of the annealing separator, it is important that the process for producing the annealing separator be carried out under suitable conditions.
Usually, the magnesium oxide used in the conventional annealing separator is produced by calcining magnesium hydroxide at a temperature of from 800.degree. to 900.degree. C. It is desirable that the annealing separator contain no impurity, for example, chlorine and sulphur, which will cause the deterioration in quality of the grain oriented silicon steel strip. Therefore the conventional annealing separator is prepared by calcining magnesium hydroxide having a high degree of purity. This magnesium hydroxide causes the resultant annealing separator to be expensive.
The conventional annealing separator consisting of the non-calcined magnesium oxide is soluble in water, and the magnesium oxide dissolved in water is gradually converted into magnesium hydroxide so as to form a stable aqueous colloidal solution. This aqueous colloidal solution of the magnesium hydroxide is effective for stabilizing the aqueous slurry of the magnesium oxide. Also, the aqueous colloidal solution of the magnesium hydroxide promotes the covering activity of the aqueous slurry of the magnesium oxide on the surface of the silicon steel strip. Accordingly, in order to prevent only the sticking of the silicon steel strips to each other, the annealing separator may consist of magnesium hydroxide alone. However, the magnesium hydroxide applied on the surfaces of the silicon steel strips is decomposed so as to produce a large amount of water during the finish annealing operation. The production of water results in reduction of the magnetic property of the silicon steel strip.
Accordingly, the conventional annealing separator is prepared by using a type of magnesium oxide having a reduced solubility in water. This type of magnesium oxide is prepared by calcining magnesium hydroxide at a temperature of from 800.degree. to 900.degree. C. In the case where the above-mentioned type of annealing separator is suspended in water, the magnesium oxide is gradually dissolved in water and, then, the dissolved magnesium oxide is converted into magnesium hydroxide. Several hours after the contact of the magnesium oxide with water, the entire amount of the magnesium oxide is dissolved in the water and converted into magnesium hydroxide. Therefore, when this type of annealing separator is applied onto the silicon steel strip, the content of water in the annealing separator on the silicon steel strip surface changes with the lapse of the storing time of the slurry. This change causes the magnetic property of the resultant annealed silicon steel strip to be changed. In the conventional process, in order to hinder the hydration of the magnesium oxide into the magnesium hydroxide, the aqueous slurry of the annealing separator is stored at a low temperature of from 10.degree. to 20.degree. C. However, the operation for maintaining the temperature of the aqueous slurry at a fixed level causes the application operation of the aqueous slurry of the annealing separator to be complicated and the efficiency of the application operation to be reduced.